1. Field of the Invention
The present invention relates to an amplifier device, and particularly to an amplifier device which reduces distortion and allows gain to be designed easily when used for an electret condenser microphone.
2. Description of the Related Art
To perform impedance conversion and amplification of an electret condenser microphone (ECM, below), a junction field effect transistor (J-FET, below) or an amplifier integrated circuit device has been used, for example. This technology is described for instance in Japanese Patent Application Publication Nos. 2003-243944 and Hei 5-167358.
An amplifier integrated circuit device (Japanese Patent Application Publication No. Hei 5-167358, for example) has an advantage in that gain can be appropriately selected according to its circuit constant and a higher gain can generally be achieved than a case of using a J-FET. However, the amplifier integrated circuit device has problems that the circuit configuration is complicated and the cost is high.
Moreover, since noise included in an input is also amplified in the amplifier integrated circuit device, S/N, which is an index of sound quality, does not improve by changing the gain. Furthermore, since noise occurs from both a resistor and a semiconductor, the amplifier integrated circuit device, having a complicated circuit configuration, has more noise sources than a J-FET, having a simple configuration, and consequently has lower S/N in general.
By contrast, a J-FET is known to have a high input impedance, have little low-frequency noise in small-signal amplification, and has excellent high-frequency characteristics. Moreover, the J-FET has a simpler circuit configuration and is less expensive than the above-described amplifier integrated circuit device. Accordingly, the amplifier integrated circuit device is required when high sensitivity is regarded as important, while the J-FET is generally used when the sensitivity of the J-FET is sufficient.
However, when only the J-FET is used, a problem arises that an output is not sufficiently amplified to result in a low gain. Since the microphone sensitivity of the ECM affects the AC component of the output voltage Vout amplified by the amplifier device as described above, a higher gain is more desirable to improve the microphone sensitivity. To increase the gain, an increase in the area of the J-FET (cell size) is effective. However, an increase in the area of the J-FET leads to an increase in an input capacitance Cin of the J-FET. This increases an input loss, leading to a decrease in S/N.